When investigating problems with an open display case, a technician must not only look at the mechanical refrigeration circuit but also at the air curtain as the possible source of the problem. When an air curtain becomes disturbed, warm store air can enter the case, causing the temperature of the case and its product to rise.

Many factors can disrupt an air curtain. As a technician, you need to identify the source and correct the problem. Always measure the discharge air velocity of the air curtain and look for sources of the disturbance. By doing this, you should be able to discover most problems associated with these air curtains.

First, of course, you will want to verify that there is an air curtain disturbance. You can do this in one of two ways:

1. Place a thin plastic bag in front of the case and the air curtain. (The bags used by the supermarkets often work very well.) If the bag is drawn into the case, so will the store air. The cause needs to be identified.

2. Smoke the case. By using a smoke candle to introduce smoke into the return air grille, you can easily see if the air curtain is intact or if it's being disturbed. Smoke candles can burn from 30 to 60 seconds and can be found at most HVACR supply houses. A 30-second burn is usually sufficient to test an air curtain.

Potential Causes

To start the diagnostic process, ask yourself these things:

  • What is the velocity of the air as it falls in front of the case?

    The velocity must be sufficient to keep the air curtain intact. If the air velocity is not adequate, it may cause store air to enter the case. Problems with the velocity of the air curtain can usually be associated with either an inoperative fan or an iced section of the evaporator coil.

    The velocity of the air curtain can be measured at the supply outlet at the top of the case. It can then be compared to the manufacturer's guidelines. It is important to obtain the correct velocity from the case manufacturer, as they may vary from case to case and from manufacturer to manufacturer.

    As a general guideline, medium-temperature, single-deck cases should have an approximate discharge velocity of 120 to 140 feet per minute (fpm). Single-deck, low-temperature cases should have a discharge velocity of approximately 200 to 250 fpm. Multi-deck, medium-temperature cases should have a discharge velocity of approximately 170 to 400 fpm. (This depends on the application and manufacturer.)

    The primary air curtain of a multi-deck, low-temperature case should have a discharge velocity of approximately 550 fpm. The secondary air curtain should have a velocity of approximately 400 fpm. The ambient air curtain should be 250 fpm.

  • Is an outside air current being directed into the case?

    Many times the source of this disturbance is a heating or air conditioning diffuser. If a supply diffuser is directed toward a case it could disrupt the air curtain. Generally, velocities of over 20 fpm near the air curtain are considered excessive.

  • How is the product loaded in the case?

    Each case is normally marked with a load limit line. If this line is exceeded, product may be in the path of the air curtain, causing store air to enter the case. Therefore, product should not be loaded beyond the load limit.

    Troubleshooting the air curtain on open display cases can be difficult at times. Don't jump to early conclusions as to the cause of the problem.

    Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or joe@rhvactools.com.

    Publication date: 03/01/2004